In a land based communication network, integrated services digital networks (ISDNs) are becoming popular because the ISDN can flexibly cope with the increasing number of subscribers and non-telephone services such as facsimile and data transmissions.
On the other hand, mobile communication systems which provide communication services to mobile stations such as mobile telephones and portable telephones are also becoming popular. In addition, in order to cope with the rapidly increasing number of subscribers and to provide various services similar to those provided by the fixed communication networks, the digital system is expected to replace the existing analog system.
According to the digital mobile communication system, it is possible to carry out various kinds of processings in the transmission path with respect to the transmitting information, and the transmission characteristic of the radio line is satisfactory regardless of the information source. Furthermore, the digital mobile communication system provides an improved privacy of the communication.
FIG. 1 generally shows an example of a conventional digital mobile communication system. In FIG. 1, base stations (CS.sub.1 through CS.sub.N-1) 51.sub.1 through 51.sub.N-1 are coupled to a public telephone network (PSTN) via a private branch exchange (PBX) 52. A base station (CS.sub.N) 51.sub.N is coupled to an integrated services digital network (ISDN) via a network (local network) which is operated by a communication company.
The base stations 51.sub.1 through 51.sub.N-1 form a private mobile communication system which is operated by a person who installs these base stations 51.sub.1 through 51.sub.N-1 and the private branch exchange 52. This private mobile communication system provides communication services to portable mobile stations (PS.sub.1 and PS.sub.2) 53.sub.1 and 53.sub.2 via digital radio lines which employs the TDMA. The base station 51.sub.N forms a public mobile communication system which is operated by a person who installs the base station 51.sub.N. This public mobile communication system provides a communication service to a portable mobile station (PS.sub.3) 53.sub.3 via a digital radio line.
Of course, each of the base stations 51.sub.1 through 51.sub.N can provide similar communication services to all portable mobile stations located within the respective service areas, and is not limited to providing the communication services to the specific portable mobile stations mentioned above.
In the digital mobile communication system having the construction described above, it is necessary to suppress the blocking probability by providing a large number of speech channels in each radio zone of the plurality of sub systems. For this reason, only one or two channels are allocated as the common control channel or channels for all of the base stations and mobile stations. The control channel is used to exchange control information between the base and mobile stations depending on the calling and receiving operations of each mobile station.
Accordingly, the common control channel must not be used continuously by one particular base station or mobile station. For example, in the down-control channel which is used when the base station accesses the mobile station, an intermittent transmission system is employed whereby each base station makes only one transmission for every period of 125 ms, and the transmission is made in time slots each having a width of 625 .mu.s as shown in FIG. 2. On the other hand, in the up-control channel which is used when the mobile station accesses the base station, a slotted aloha system which is synchronized to the down-control channel is used as shown in FIG. 2.
In this specification, the up-control channel refers to the control channel which is used when the transmission is made from the mobile station to the base station, and the down-control channel refers to the control channel which is used when the transmission is made from the base station to the mobile station. The terms "up" and "down" are used similarly throughout this specification.
For example, as shown in FIG. 3 the mobile station 53.sub.1 which is located in a zone a carries out an intermittent receiving operation with the same timing as the intermittent transmission timing of a base station 51.sub.a which is located in this zone a. In addition, by the receiving operation of the mobile station 53.sub.1, the mobile station 53.sub.1 monitors a call generated thereto while entering the control information which is transmitted from the base station 51.sub.a. In addition, the mobile station 53.sub.1 exchanges the control information with the base station 51.sub.a via the control channel according to a predetermined control procedure which confirms to the generated call.
The mobile station 53.sub.1 further sets an oscillation frequency of an internal frequency synthesizer to a frequency corresponding to a speech channel which is allocated by the base station 51.sub.a, and makes the call via the time slot indicated by the hatchings in FIG. 2.
The mobile station 53.sub.1 also monitors the deterioration of the speech quality depending on the received electric field level of the speech channel or the like, and as shown in FIG. 3, detects a control channel with the best transmission quality based on the received electric field level of the control channel which is controlled by the base station in each zone which is adjacent to the zone a, the received error rate of the control information and the like if the mobile station 53.sub.1 detects that the mobile station 53.sub.1 is moving outside the zone a.
For example, a base station 51.sub.b which controls the control channel which is detected by the mobile station 53.sub.1 as having the best transmission quality allocates an free speech channel to the mobile station 53.sub.1. Hence, the mobile station 53.sub.1 continues the call by switching the oscillation frequency of the internal frequency synthesizer to the frequency which specifies the free speech channel.
Compared to the mobile communication system in which the hand over is controlled primarily by the base stations and the switching system, that is, the network, the method of controlling the hand over primarily by the mobile station can considerably reduce the load on the network.
However, according to the conventional mobile station, the process to search the zone to which the hand over of the channel in use is to be made starts after the mobile station detects deterioration of the speech quality in the down-speech channel as the mobile station moves. For this reason, even though the call is being made, the oscillation frequency of the internal frequency synthesizer of the mobile station must be frequently switched to the frequency which specifies the control channel of each adjacent zone until the search process is completed. Accordingly, the speech quality of the call in which the hand over of the channel in use is made greatly deteriorates because the hit time of the call becomes long.
In order to eliminate the above described problem, it is conceivable to provide in the mobile station a frequency synthesizer which can switch its oscillation frequency at an extremely high speed for every time slot of the speech channel, and to dynamically switch and monitor the control channel in each time slot during the call preceding the detection of the deterioration of the speech quality. However, at the present, even the frequency synthesizer having the fastest oscillation frequency switching speed requires a switching time of approximately 1 ms which corresponds to approximately two time slots shown in FIG. 2. Therefore, this conceivable method cannot be realized in actual practice.